Dietary carbohydrate composition

Introduction

While a formal definition of a carbohydrate can be considered somewhat difficult, one commonly accepted by chemists is that carbohydrates are "polyhydroxy aldehydes, ketones, alcohols, acids, their simple derivatives and their polymers having polymeric linkages of the acetal type" (31). Carbohydrates are further classified according to their degree of polymerization (DP) as: sugars (mono- and disaccharides), oligosaccharides (contain three to nine monosaccharide units), and polysaccharides (contain ten or more monosaccharide units) (32).

Carbohydrates play a major role in human diets, comprising some 40-75% of energy intake. Their most important nutritional property is digestibility in the small intestine. In terms of their physiological or nutritional role, they are often classified as available and unavailable carbohydrates. Available carbohydrates are those that are hydrolyzed by enzymes of the human gastrointestinal system to monosaccharides that are absorbed in the small intestine and enter the pathways of carbohydrate metabolism. Unavailable carbohydrates are not hydrolyzed by endogenous human enzymes, although they may be fermented in the large intestine to varying extents.

The carbohydrates in foods of greatest importance are shown in Table 5 (32,33). Small amounts of other carbohydrates can be detected in some foods but these are of little overall significance. These include maltose, commonly formed from hydrolysis of starch and found in starch hydrolyzates used as food ingredients; galactose from fermented dairy products; and pentoses, such as xylose and arabinose, from fruits.

TABLE 5 The most important carbohydrates in foods

Monosaccharides	Glucose, Fructose
Disaccharides	Sucrose, Lactose
Oligosaccharides	Raffinose, Stachyose, Fructo-oligosaccharides
Polysaccharides	Cellulose, Hemicelluloses, Pectins, b -Glucans, Fructans, Gums, Mucilages, Algal polysaccharides
Sugar alcohols	Sorbitol, Mannitol, Xylitol, Lactitol, Maltitol

Monosaccharides

Glucose (also called dextrose) is found in varying amounts in honey, maple syrup, fruits, berries, and vegetables. Glucose is often formed from the hydrolysis of sucrose, as in honey, maple sugar, and invert sugar. It is also present in foods containing starch hydrolysis products, such as corn syrups and high-fructose corn syrups. The amount of glucose contained in these starch hydrolysates depends on the method used in their preparation, e.g. acid conversion, acid-enzyme conversion, or enzyme conversion (34). Small amounts of glucose are also found in maltodextrins and corn syrup solids, commonly used as ingredients in food products.

The physicochemical properties of glucose that are important in its use as a food ingredient include flavour, sweetness, hygroscopicity, and humectancy (35). It is about 70-80% as sweet as sucrose. Since glucose is a reducing sugar, i.e. contains a carbonyl function at the C-1 position, it readily undergoes the Maillard or browning reaction with amino acids. This behaviour is responsible for the golden-brown colour of bread crusts and for the caramel colour and flavour observed in certain other foods. If the reaction is sufficiently extensive, dark brown colours and altered flavours can result in food products.

Fructose is present in honey, maple sugar, fruits, berries and vegetables. Fructose is often present from the hydrolysis of sucrose, as in honey, maple sugar, and invert sugar. It may also be present in food products, such as soft drinks, bakery products, and candies from the use of invert sugar, crystalline fructose or high-fructose corn syrups (HFCS). HFCS are often used as a sweetening agent, particularly in carbonated soft drinks. Fructose is 140% sweeter than sucrose (35). As a keto-hexose, fructose is very reactive with amino acids in the Maillard or browning reaction.

Disaccharides

Sucrose (a -D-glucopyranosyl b -D-fructofuranoside or b -D-fructofuranosyl a -D-glucopyranoside) is a nonreducing sugar and is the major disaccharide in most diets. It is present in honey, maple sugar, fruits, berries, and vegetables. It may be added to food products as liquid or crystalline sucrose or as invert sugar (if not completely inverted to fructose and glucose). It is commercially prepared from sugar cane or sugar beets. Sucrose can provide a number of desirable functional qualities to food products including sweetness, mouth-feel, and the ability to transform between amorphous and crystalline states (35). Sucrose often cannot be easily replaced by other sweeteners due to its lack of aftertaste, browning, and gummy mouth-feel and its characteristic body, viscosity and sweetness profile. This is particularly true in baked products where equivalent replacement, particularly with high intensity sweeteners, often results in a product with decreased textural characteristics and consumer acceptance. Sucrose and invert sugar are used in many food products including ice cream, baked goods, desserts, confections, intermediate-moisture foods, and soft drinks. Its use in soft drinks has decreased because of the increased usage of high-fructose corn syrups due to availability and lower costs.

Lactose (4-O-b -D-galactopyranosyl-D-glucose), a reducing sugar also known as milk sugar, occurs in milk and milk products. It may also occur in food products that contain dairy products as ingredients, such as doughnuts, wafer cookie bars, breakfast bars, and hamburger buns. Whey is used as an ingredient in foods and is high in lactose content. Lactose crystallizes easily and is often responsible for the grittiness encountered in ice cream when crystallization is not inhibited. Lactose serves as an energy source for infants during the nursing period. Lactose is hydrolyzed in the small intestine by the enzyme lactase to galactose and glucose which are then absorbed.

Oligosaccharides

Oligosaccharides are not widely occurring or of great importance in foods and food products, except for a series of galactosylsucroses (often designated as a -galactosides) and fructo-oligosaccharides. The galactosylsucrose family of oligosaccharides include raffinose (a trisaccharide), stachyose (a tetrasaccharide), and verbascose (a pentasaccharide). In vegetables, such as peas, beans, and lentils, the content of these oligosaccharides can range from five to eight percent on a dry matter basis (36). Raffinose, stachyose, and verbascose are not digested in the small intestine by human gastrointestinal enzymes. They are passed into the large intestine where they are fermented by intestinal microflora with the production of gas. It is this behaviour that produces the flatulence for which the consumption of beans is noted. Enzyme preparations are commercially available which can be taken with meals to reduce the tendency for flatulence production by promoting the hydrolysis of these oligosaccharides to constituent monomers which are absorbed.

Fructo-oligosaccharides occur in wheat, rye, triticale, asparagus, onion, and Jerusalem artichoke and a number of other plants. Fructo-oligosaccharides and higher molecular weight fructans can comprise 60-70% of the dry matter in Jerusalem artichokes. Fructo-oligosaccharides have been commercially prepared by the action of a fructofuranosyl furanosidase from Aspergillus niger on sucrose. They are about 30% as sweet as sucrose, have a taste profile similar to sucrose, are stable at pH values above 3 and at temperatures up to 140°C (37). Since fructo-oligosaccharides are non-reducing oligosaccharides, they do not undergo the Maillard browning reaction.

Polisaccharides

Starch

Starch is the most important, abundant, digestible food polysaccharide. It occurs as the reserve polysaccharide in the leaf, stem (pith), root (tuber), seed, fruit and pollen of many higher plants. It occurs as discrete, partially-crystalline granules whose size, shape, and gelatinization temperature depend on the botanical source of the starch. Common food starches are derived from seed (wheat, maize, rice, barley) and root (potato, cassava/tapioca) sources (see Table 6). Starches have been modified to improve desired functional characteristics and are added in relatively small amounts to foods as food additives.

Starch is a homopolysaccharide composed only of glucose units and consists of a mixture of two polymers, amylose and amylopectin, whose glucopyranosyl units are linked almost entirely through a -D-(1->4)-glucosidic bonds. Amylose shows many of the properties of a linear polymer and has historically been considered to be a linear polymer with a degree of polymerization of approximately 1000 or less. However, it is now known that amylose contains a limited amount of branching involving a -D-(l->6)-glucosidic linkages at the branch points. Amylopectin is a high molecular weight, highly branched polymer containing about 5-6% of a -D-(1->6)-glucosidic linkages as the branch points. The average chain length is 20 to 25 units with an average degree of polymerization in the thousands, and molecular weight in the millions.

While the manner in which amylose and amylopectin are organized to form the starch granule is not clearly understood, the granule is partially crystalline exhibiting an x-ray diffraction pattern and birefringence. Most common cereal starches contain 20-30% amylose. Waxy starches (maize, rice, sorghum, barley) have no amylose and contain essentially 100% amylopectin. The first example of a waxy wheat starch has recently been reported from Japan. High-amylose starches (maize, barley) having 50-70% amylose are available. Waxy and high-amylose starches differ from normal starches in some properties that make them of use in certain food products. A number of double and triple maize starch mutants are being investigated to determine whether they have unique or desirable physicochemical and/or functional properties that would make them of use in selected food products.

TABLE 6 Some properties of whole granular starches

Source	Gelatinization Temperature Range, °C	Granule Shape	Granule Size (mm)	Iodine Binding Capacity (g I2/100g)	Amylose Content (%)
Barley	51-60	Round or lenticular	20-25 2-6	4.3	22
Triticale	55-62	Round	19 (2-35)	-	23-24
Wheat	58-64	Lenticular or Round	20-35 2-10	5.0	26 (23-27)
Rye	57-70	Round or lenticular	28 (12-40)	5.5	27
Oats	53-59	Polyhedral	5-10	5.1	23-24
Potato	59-68	Oval	40(15-100)	4.5	23
Maize	62-72	Round or polyhedral	15 (5-25)	5.3	28
Waxy maize	63-72	Round	15 (5-25)	0.1	1
Broad bean	64-67	Oval	30	4.5	24
Sorghum	68-78	Round	15-35	-	25 (23-28)
Rice	68-78	Polygonal	3-8		17-19* 21-22**
High amylose	67-80	Round	25	ca. 10.5	52
Maize		Irregular sausage shaped
Peas smooth	55/70	Reniform*** (simple)	5-10	6.7	33-36
wrinkled	>99	Reniform*** (cmpd)	30-40	14.7	71-76

^* japonica; ^** indica; ^*** kidney-shaped

Starch granules are not water soluble but easily hydrate in aqueous solution, swelling about 10% in volume. When an aqueous suspension of granules is heated, additional swelling occurs until a temperature is reached where there is a transition from organization to disorganization. This is known as the gelatinization temperature and normally occurs over a range of about 10°C. The digestion of starch by a-amylase is greatly enhanced by gelatinization. Upon further heating (pasting or cooking), swelling continues and the amylose and portions of the amylopectin leach from the granule producing a viscous suspension. Cooling of this suspension leads to the formation of a gel. With further time, realignment of the linear chains of amylose and the short chains of amylopectin can occur in the process known as retrogradation. In food products based on starch gels, this can lead to liquid being expressed from the gel in the phenomenon known as syneresis, which is generally an undesirable occurrence. Retrogradation is most rapid with amylose and much slower and more incomplete with amylopectin due to the short chain length of its branches.

Modified starches

Many starches do not have the functional properties needed to impart or maintain desired qualities in food products. As a result, some starches have been modified to obtain the functional properties required. The types of modified starches, also known as starch derivatives, and some of their functional properties are listed in Table 7 (38).

The starches most commonly modified for commercial use are those from normal maize, tapioca, potato, and waxy maize. Modified starches are used to improve viscosity, shelf stability, particulate integrity, processing parameters, textures, appearance and emulsification. While virtually all of the different types of modified starches find use in the food industry, substituted and cross-linked starches are particularly important. These two types of modified starches are produced by reactions in which a small number of hydroxyl groups on the glucose units of amylose and amylopectin, mostly in amorphous regions and on the surface of the granule, are modified without destroying the granular nature of the starch.

Substituted starches are produced by etherification or esterification. This reduces the tendency of chains to realign (retrograde) following gelatinization of starch during heat processing. Substitution lowers the gelatinization temperature, gives freeze-thaw stability, increases viscosity, increases clarity, inhibits gel formation, and reduces syneresis.

Cross-linked starches are produced by introducing a limited number of linkages between the chains of amylose and amylopectin using difunctional reagents. Cross-linking essentially reinforces the hydrogen bonding occurring within the granule. It increases gelatinization temperature; improves acid stability, heat stability and shear stability; inhibits gel formation; and controls viscosity during processing.

TABLE 7 Types of modified starches

Modified starch		Functional properties
Bleached		Oxidized - Lighten colour, sterilize
Converted		Hydrolyzed - Reduces viscosity
	(a) Thin boiling	Fluidity
	(b) Dextrins	Dry roasting
	(c) Oxidized	Creaminess, short body
Crosslinked		Strengthens granule
		Increases viscosity
		Tolerance to acidity
		Yields shear to resistance
		Heat penetration
Stabilized		Resist retrogradation
		Low temperature stability

Dietary fibre

Dietary fibre has been considered to be composed of non-starch polysaccharides plus lignin plus resistant oligosaccharides plus resistant starch. Since lignin is not a carbohydrate, it will not be discussed. Dietary fibre occurring in foods and food products can be considered to consist of cellulose, hemicelluloses, pectic substances, hydrocolloids (gums and mucilages), resistant starches, and resistant oligosaccharides.

Cellulose, the major cell wall structural component in plants, is an unbranched linear chain of several thousand glucose units with b -D-(1->4)-glucosidic linkages. Cellulose's mechanical strength, resistance to biological degradation, low aqueous solubility, and resistance to acid hydrolysis result from hydrogen bonding within the microfibrils (39). There is a portion (10-15%) of the total cellulose, referred to as "amorphous," that is more readily acid hydrolyzed. Controlled acid hydrolysis of the amorphous fraction yields microcrystalline cellulose. Cellulose has been used as a bulking agent in food due to its water-absorbing ability and low solubility. Some of the early dietary fibre ingredient sources were based on cellulose powders or microcrystalline cellulose. Cellulose is not digested to any extent by the enzymes of the human gastrointestinal system.

Hemicelluloses may be present in soluble and insoluble forms and are comprised of a number of branched and linear pentose- and hexose-containing polysaccharides. In cereal grains, soluble hemicelluloses are termed "pentosans." Hemicelluloses are of much lower molecular weight than cellulose. Component monosaccharide units may include xylose, arabinose, galactose, mannose, glucose, glucuronic acid, and galacturonic acid.

Mixed linkage b -glucans, the (1->3)(1->4)- b -D-glucans, have generated considerable interest in recent years due to their physiological response as soluble dietary fibre. While these glucans are found in relatively small quantities in wheat, they are a major component of cell-call material in barley and oats. These glucans form viscous aqueous solutions and have been shown to be effective in reducing serum cholesterol concentrations (40). Oat bran, a rich source of b -D-glucan, has been incorporated into many food products, particularly cereals, as a source of the soluble fibre that has been touted for cholesterol reduction.

Both soluble and insoluble hemicelluloses play important roles in food products, the former functioning as soluble and the latter as insoluble fibre. They are characterized by their ability to bind water and hence serve as bulking agents. The presence of acidic components in some hemicelluloses impart the capacity to bind cations. Hemicelluloses are fermented to a greater extent than cellulose in the colon.

Pectins find widespread use in foods such as jams and jellies because of their ability to form stable gels. Completely esterified pectins do not require the addition of acid or electrolyte to form gels. The presence of calcium salts enhances the gelling capacity and decreases the dependence on pH and sugar concentration. Pectic substances are of importance as a component of dietary fibre because of their ion-exchange properties, due to the presence of the galacturonic acid units, and gelling (viscosity enhancing) properties.

Hydrocolloids (gums, mucilages) are used in small amounts in food products for their thickening (viscosity increasing), gelling, stabilizing, or emulsifying ability. They are derived from seaweed extracts, plant exudes, seeds, and microbial sources.

Resistant starch

While starch was long thought to be completely digested, it is now recognized that there is a portion (resistant starch) which resists digestion, passes into the lower intestine, and is fermented there. Resistant starch has been defined as "the sum of starch and products of starch degradation not absorbed in the small intestine of healthy individuals" (41). Three types of resistant starch have been identified (41,42):

1. RS1 - Physically trapped starch: These starch granules are physically trapped within a food matrix so that digestive enzymes are prevented or delayed from having access to them. This can occur in whole or partly ground grains, seeds, cereals, and legumes. The amount of type 1 resistant starch will be affected by food processing and can be decreased or eliminated by milling.

2. RS2 - Resistant starch granules: Certain raw (native) starch granules, such as potato and green banana, are known to resist attack by a -amylase. This is probably related to the crystalline nature of the starch (i.e., crystalline regions of the starch granule are less susceptible to attack by acid and enzymes than the amorphous regions). Gelatinization normally occurs during cooking and food processing, although the extent is dependent on the moisture content of the food product and may not be complete in water-limited systems (e.g. sugar cookies). Gelatinized starch is much more rapidly digested by enzymes than is raw starch. Gelatinized potato and green banana starch are digested by a -amylases.

High-amylose maize starches have high gelatinization temperatures, requiring temperatures that are often not reached in conventional cooking practices (154-171°C) before the granules are completely disrupted. As a result, undigested starch granules have been observed in the effluent from ileostomates fed a meal containing high amylose maize (41). These starches offer an opportunity to manipulate the amount of resistant starch present in food products.

3. RS3 - Retrograded starch: The amylose and amylopectin components of starch undergo the process of retrogradation in a time dependent process after starch has been gelatinized/cooked. The rate at which amylose retrogrades is much higher than that for amylopectin which has much shorter chain lengths. Amylose can be retrograded to a form that resists dispersion in water and digestion with a-amylase (41). This form of resistant starch can be generated during food processing.

There is currently great interest in resistant starch because of is potential use as a food ingredient to increase the dietary fibre content of foods and also because it may be possible to manipulate the amount of resistant starch in food products through processing conditions.

Sugar alcohols (alditols, polyols)

Monosaccharides and disaccharides in which the aldose and ketose functional groups have been reduced to hydroxyl groups are known as sugar alcohols (alditols, polyols). Sugar alcohols, such as sorbitol, occur in small amounts in fruits. Due to their physicochemical properties and relative sweetness, sugar alcohols have found use as bulk sweeteners. Xylitol has a negative heat of solubility which produces a cooling sensation when used in products such as chewing gum. The sugar alcohols undergo limited absorption in the small intestine, and this can lead to laxative effects in many people when large amounts (50 grams or more at one time) are consumed. There appear to be no other health risks associated with the consumption of sugar alcohols.